1. Field of the Invention
The present invention relates to an extruder for plastics-processing, and, more particularly, to a plastics-processing apparatus which has a combined performance of extrusion and injection.
2. Description of Prior Art
As is well known in the art, the extruders for plastics-processing are machines for manufacturing plastics products with uniform cross-section, such as film, sheet, pipes, angulars or rods, in continuous lengths. The injection molding machines are, on the other hand, plastics-processing machines for producing three-dimentional mouldings one by one by intermittently injecting molten polymer into molds.
Comparison between the machines of these two types can be tabulated as to their performance, as follows:
Performance Comparison between Extruders and Injection Molding Machines __________________________________________________________________________ Extruders Injectors Productivity Great Small Products lengths having the Three-dimentional same cross-section Moldings Material discharge method Screw rotation Plunger stroke Plasticizing capacity Great Small Plasticizing property Low temperature High temperature Uniform Non-uniform Mixing property Excellent Poor Dispersing property Excellent Poor Processing temperature Low High Product cooling Rapid Slow Stay period Short Long Melt stagnation None Occurs Maintenance & control Easy Difficult Cost High Low __________________________________________________________________________
The above differences in performance are considered to come from the basic difference in processing operation.
More specifically, since the extruders are equipped with a long screw, the polymer can be heated gradually with a gentle temperature gradient during the period from the time when it is fed to the hopper to the time when it is discharged from the outlet of the nozzle. As a result, there is little cross-sectional temperature difference, that is, the temperature difference between the material, which is sticks to the inner surface of the cylinder, and the material which is sticks to the outer surface of the screw. As compared with this long screw extrusion, however, the injection molding machines with a short screw (or plunger) have such a construction as to inevitably heat the polymer rapidly within a short period of time, so that great difference in the cross-sectional temperature will result. This may lead to insufficient plasticizing and mixing of the polymer.
Since, moreover, the long screw extrusion can process the material at a considerably lower temperature than the short screw injection, the cooling time period required can be shortened accordingly, and the possible thermal deterioration of the material can be effectively prevented.
Since the long screw extrusion can enjoy great efficiency in mixing, it should also be appreciated as another advantage that products of minute cell-structure with improved foaming ratios can be obtained if applied to foam molding, and that uniformly colored products can be obtained by using dry-color.
Theoretically, it is of course possible to install long screws in the conventional injection molding machines. In this instance, however, a complicated hydraulic mechanism of large scale is required, which makes this impracticable.
For these reasons, extruders can be said to have greater productivity and higher performance at lower cost than injection molding machines.
When it is intended to afford the injection molding function to conventional extruders, several impediments are encountered. In order to apply the extruders to intermittent injection, a considerable duration of time is required until a proper level of injection pressure is obtained after the screw is started. In addition, the screw, which is rotating with a large torque, cannot practically be stopped due to its inertia, even if the driving motor is stopped after the polymer injection. When, moreover, the nozzle is shut simultaneously with the stop of screw rotation, abnormally high pressure is induced in the vicinity of the tip of the closed nozzle. The high pressure thus induced will bring about leakage of the polymer from the nozzle tip or, in some case, will invite machine breakage.